The present invention relates in general to NMR tomography as it is used in medicine for examining patients. In this case, the present invention relates particularly to an NMR tomography machine with reduced vibrations of machine components that negatively influence many aspects of the overall system.
NMR is based on the physical phenomenon of nuclear spin resonance and has been used successfully as an imaging method for over 15 years in medicine and in biophysics. In this method of examination, the object is exposed to a strong, constant magnetic field. This aligns the nuclear spins of the atoms in the object, which were previously oriented irregularly. Radio-frequency waves can now excite these “ordered” nuclear spins to a specific oscillation. In NMR, this oscillation generates the actual measuring signal that is picked up by way of suitable receiving coils.
Due to the use of inhomogeneous magnetic fields, generated by gradient coils, it is possible in this case to code the measurement object spatially in all three spatial directions. The method permits a free choice of the layer to be imaged, as a result of which it is possible to take tomographic images of the human body in all directions. NMR as laminography in medical diagnostics is distinguished first and foremost as a “non-invasive” method of examination by a versatile contrast capability. NMR has developed into a method far superior to x-ray computer tomography (CT) because of the excellent ability to display the soft tissue. Currently, NMR is based on the application of spin echo and gradient echo sequences that permit an excellent image quality with measuring times in the range of seconds to minutes.
Continuous technical development of the components of NMR machines, and the introduction of high-speed imaging sequences has opened up ever more fields of use for NMR in medicine. Real time imaging for supporting minimally invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are only a few examples.
The basic design of one of the central parts of such an NMR machine is illustrated in FIG. 2. It shows a superconducting basic field magnet 1 (for example, an axial superconducting air-coil magnet with active stray field screening) which generates a homogeneous magnetic basic field in an inner space. The superconducting basic field magnet 1 comprises the inner space coils which are located in liquid helium. The basic field magnet is surrounded by a two-shell tank which is made from stainless steel, as a rule. The inner tank, which contains the liquid helium and serves in part also as winding body for the magnet coils is suspended at the outer tank, which is at room temperature, via fiber-glass-reinforced plastic rods which are poor conductors of heat. A vacuum prevails between inner and outer tanks. The inner and outer tanks are referred to as a magnet vessel.
The cylindrical gradient coil 2 in the inner space of the basic field magnet 1 is inserted concentrically into the interior of a support tube by way of support elements 7. The support tube is delimited externally by an outer shell 8, and internally by an inner shell 9. The function of the layer 10 will be explained below.
The gradient coil 2 has three-part windings that generate a gradient field, which is proportional to the current impressed in each case, and are spatially perpendicular to one another in each case. As illustrated in FIG. 3, the gradient coil 2 comprises an x coil 3, a y coil 4 and a z coil 5 which are respectively wound around the coil core 6 and thus generate a gradient field, expediently in the direction of the Cartesian co-ordinates x, y and z. Each of these coils is fitted with a dedicated power supply unit in order to generate independent current pulses with accurate amplitudes and timing in accordance with the sequence programmed in the pulse sequence controller. The required currents are at approximately 250 A.
Located inside the gradient coil is the radio-frequency coil (RF resonator or antenna). Its task is to convert the RF pulses output by a power transmitter into an alternating electromagnetic field for the purpose of exciting the atomic nuclei, and subsequently to convert the alternating field emanating from the preceding nuclear moment into a voltage supplied to the reception path.
Since the gradient switching times are to be as short as possible, current rise rates of the order of magnitude of 250 kA/s are necessary. In an exceptionally strong magnetic field such as is generated by the basic field magnet 1 (typically between 0.2 and 1.5 Tesla), such switching operations are associated with strong mechanical vibrations because of the Lorentz forces occurring in the process. All system components which are mechanically coupled to the gradient system (housing, covers, tank of the basic field magnet and magnet casing, respectively, RF body coil etc.) are excited to forced vibrations.
Since the gradient coil is generally surrounded by conductive structures (for example, a magnet vessel made from stainless steel), the pulsed fields start in these eddy currents which exert force effects on these structures due to interaction with the basic magnetic field, and likewise excite these structures to vibrations.
These vibrations of the various NMR machine components act negatively in many ways on the NMR system:    1. Strong air-borne noise is produced, which constitutes an annoyance to the patient, the operating staff and other persons in the vicinity of the NMR system.    2. The vibrations of the gradient coil and of the basic field magnet, and their transmission to the RF resonator and the patient couch in the inner space of the basic field magnet and/or the gradient coil, are expressed in inadequate clinical image quality which can even lead to misdiagnosing (for example, in the case of functional imaging, fMRI).    3. If the vibrations of the outer tank are transmitted to the inner tank via the GRP rods, or the superconductor itself is excited to vibrate, increased helium damping occurs—in a way similar to in an ultrasonic atomizer—in the interior of the tank, thus necessitating the subsequent supply of a correspondingly larger quantity of liquid helium, and this entails higher costs.    4. High costs arise also due to the need for a vibration-damping system set-up—similar to an optical table—in order to prevent transmission of the vibrations to the ground, or vice versa.
In the prior art, the transmission of vibrational energy between the gradient coil and the further components of the tomograph (magnet vessel, patient couch, etc.) is counteracted by the use of mechanical and/or electromechanical vibration dampers. It is customary to make use of passively acting components, for example, rubber bearings, or possibly piezo-actuators integrated in the gradient coil, which permit active counter control in regulated operation and thus reduce the amplitude of vibration of the gradient coil. Vibrations of the magnet vessel are usually mechanically damped with respect to the gradient coil by bolsters.
The following passive measures are also usually undertaken in order to reduce the vibrations:
encapsulation of vibration source
use of thick and heavy materials
damping layers (for example, tar) applied from “outside”.
In particular, the noise production path over the interior of the NMR machine, i.e., the production of noise by vibration of the gradient coil and transmission of the noise to the support tube located in the gradient coil (8, 9 FIG. 2), which emits this noise inward to the patient and the inner space 15, is blocked in accordance with U.S. Pat. No. 4,954,781 by a damping viscoelastic layer 10 (FIG. 2) in the double-ply interior of the support tube.
Furthermore, it is known to achieve the above-named blocking of the noise production path by inserting sound-absorbing “acoustic foams” into the region between support tube and gradient coil.
Thus, German Published Patent Application DE 197 22 481 A1 discloses an NMR device that has, between a magnet assembly and a gradient coil assembly, a passive acoustic damping device which is intended, firstly, to dampen vibrations and, secondly, to stiffen the gradient coil assembly. Materials named are: foam, air or other gases, liquid, gaseous or pourable insulating materials.
According to the '481 application, the action of the noise-reducing device is based essentially on the fact that the total deformation of the gradient coil assembly, and thus also the noise emission, are reduced with increasing stiffness. Full-area contact between two facing surfaces of the magnet assembly and gradient coil assembly is proposed as being particularly effective.
The experimental implementation of a material that has negative stiffness and, as such, can be advantageously used in fields where damping and stiffening properties are required at a high level is disclosed in “Lakes et Al., Extreme damping in composite materials with negative-stiffness inclusions, Nature, 29 Mar. 2001, Vol. 410, No. 6828. pages 565-567” and in “Lakes: Extreme damping in composite materials with a negative stiffness phase, Physical review letters, Mar. 26, 2001, Vol. 86, No. 13, pages 2897-2900”. Both of these publications describe the physical and technical properties of the novel material, but do not disclose the technical implementation of the use of this material in a complex system such as represented by an NMR machine, for example. In general, such materials have not yet been used in NMR machines.
Nevertheless, the acoustic emission of a currently normal NMR machine continues to be very high.